High-Strength Aluminum Alloy Extruded Material That Exhibits Excellent Formability And Method For Producing The Same

ABSTRACT

An aluminum alloy is provided that is used to produce a high-strength aluminum alloy extruded material that exhibits excellent formability. The aluminum alloy consists of 0.30 to 1.00 mass % of Mg, 0.6 to 1.40 mass % of Si, 0.10 to 0.40 mass % of Fe, 0.10 to 0.40 mass % of Cu, 0.005 to 0.1 mass % of Ti, 0.3 mass % or less of Mn, 0.01 to 2.0 mass % of Zn, and 0.10 mass % or less of Zr, with the balance being aluminum and unavoidable impurities, the aluminum alloy having a stoichiometric Mg2Si content of 0.60 to 1.30 mass % and an excess Si content of 0.30 to 1.00 mass %.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No. 15/248,114 filed Aug. 26, 2016, which is a continuation of International Patent Application No. PCT/JP2015/050595, having an international filing date of Jan. 13, 2015, which designated the United States, the entirety of which is incorporated herein by reference. Japanese Patent Application No. 2014-038677 filed on Feb. 28, 2014 is also incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates to an extruded material formed of an Al—Mg—Si-based aluminum alloy that exhibits excellent hardenability during extrusion and exhibits excellent formability during press forming and the like, and a method for producing the same.

BACKGROUND ART

In recent years, automotive parts made of aluminum have been studied and put to practical use in order to reduce the weight of automobiles to improve travel performance and reduce fuel consumption from the viewpoint of global environmental protection.

An automotive structural material is required to exhibit high strength, press workability, bendability, and corrosion resistance, and a 7000 series aluminum alloy (Al—Zn—Mg-based aluminum alloy) and a 6000 series aluminum alloy (Al—Mg—Si-based aluminum alloy) have attracted attention. However, the 7000 series aluminum alloy (natural age hardening alloy) has a drawback in that working becomes difficult due to hardening when the time elapsed from extrusion to press working or bending is long. Moreover, the 7000 series aluminum alloy shows a decrease in corrosion resistance under a stress environment.

Therefore, the 6000 series aluminum alloy (artificial age hardening alloy) has been considered promising as a heat-treatable alloy that exhibits excellent corrosion resistance.

An extruded material formed of a known high-strength 6000 series aluminum alloy exhibits high tensile strength, but exhibits insufficient elongation, and easily produces cracks during press working or bending.

A water-cooling press quenching treatment is normally performed in order to obtain high strength. However, since a difference in cooling rate occurs due to the cross-sectional shape of the extruded material, the difference in thickness, and the like, the extruded material shows a non-uniform temperature distribution during cooling, and the non-uniform temperature distribution results in strain. Therefore, the dimensional accuracy deteriorates, and it is difficult to reduce the thickness of the cross-sectional profile. The degree of freedom with regard to the cross-sectional shape decreases as a result of preventing the occurrence of such strain.

The water-cooling press quenching treatment has another disadvantage in that an increase in cost occurs as compared with an air-cooling quenching treatment.

A rolled sheet material is normally used for press forming. However, the rolled sheet material is expensive since a number of steps (e.g., hot rolling and cold rolling) are required to produce the rolled sheet material.

An air-cooling press quenching treatment has an advantage in that cost can be reduced as compared with the water-cooling press quenching treatment since a simple device can be used. However, when a known alloy is used, sufficient strength or ductility may not be obtained, and formability may be insufficient.

Japanese Patent No. 5059423, Japanese Patent No. 3819263 and JP-A-2011-252212 disclose an Al—Mg—Si-based alloy that exhibits improved formability. However, the Al—Mg—Si-based alloys disclosed in Japanese Patent No. 5059423, Japanese Patent No. 3819263 and JP-A-2011-252212 are rolled materials.

Japanese Patent No. 5160930 discloses an aluminum alloy extruded material that exhibits excellent bending crush resistance and corrosion resistance. However, the aluminum alloy extruded material is designed on the assumption the aluminum alloy extruded material has a balanced composition close to the stoichiometric composition of Mg₂Si, and forced cooling is required to obtain an average cubic orientation area ratio of 15% or more.

SUMMARY OF THE INVENTION

An object of the invention is to provide an aluminum alloy extruded material that exhibits excellent hardenability that ensures that high strength can be obtained by air-cooling immediately after extrusion and artificial aging, and exhibits excellent formability (e.g., press formability) and a method for producing the same.

According to one aspect of the invention, an aluminum alloy that is used to produce a high-strength aluminum alloy extruded material that exhibits excellent formability includes 0.30 to 1.00 mass % of Mg, 0.6 to 1.40 mass % of Si, 0.10 to 0.40 mass % of Fe, 0.10 to 0.40 mass % of Cu, 0.005 to 0.1 mass % of Ti, and 0.3 mass % or less of Mn, with the balance being aluminum and unavoidable impurities, the aluminum alloy having a stoichiometric Mg₂Si content of 0.60 to 1.30 mass % and an excess Si content of 0.30 to 1.00 mass %.

According to another aspect of the invention, an aluminum alloy that is used to produce a high-strength aluminum alloy extruded material that exhibits excellent formability includes 0.30 to 1.00 mass % of Mg, 0.6 to 1.40 mass % of Si, 0.10 to 0.40 mass % of Fe, 0.10 to 0.40 mass % of Cu, 0.005 to 0.1 mass % of Ti, 0.3 mass % or less of Mn, 0.01 to 2.0 mass % of Zn, and 0.10 mass % or less of Zr, with the balance being aluminum and unavoidable impurities, the aluminum alloy having a stoichiometric Mg₂Si content of 0.60 to 1.30 mass % and an excess Si content of 0.30 to 1.00 mass %.

A high-strength aluminum alloy extruded material that exhibits excellent formability and has a structure in which crystal grains having an aspect ratio of 4.0 or more have an average grain size of 100 μm or less can be obtained by casting an aluminum alloy having the above composition to obtain a billet, extruding the billet, air-cooling the extruded product immediately after extrusion at an average cooling rate of 50 to 150° C./min, and subjecting the cooled extruded product to artificial aging.

The alloy composition was selected for the reasons described below.

Mg and Si

Mg and Si contribute to an improvement in strength through the precipitation of Mg₂Si during a heat treatment.

If the content of either or both of Mg and Si is too high, deterioration in extrudability may occur.

Therefore, the Mg content and the Si content are preferably set to 0.30 to 1.00 mass % and 0.60 to 1.40 mass %, respectively.

When the Mg content and the Si content are within the above ranges, high strength, excellent formability, and excellent extrudability can be obtained.

In the invention, the excess Si content is set to 0.30 to 1.00 mass % in order to improve the press formability and the bendability of the material while providing the material with high strength.

It is possible to improve tensile strength without decreasing formability and extrudability when the aluminum alloy has a composition in which Si is in excess. Since a decrease in ductility may occur if the excess Si content is too high, the upper limit of the excess Si content is set to 1.00 mass %. The excess Si content is preferably set to 0.30 to 0.80 mass %.

Cu

The Cu content is preferably set to 0.10 to 0.40 mass %.

Cu contributes to an improvement in strength and ductility. If the Cu content is too high, however, a decrease in corrosion resistance may occur, and formability and extrudability may be impaired.

Fe

The Fe content is preferably set to 0.10 to 0.40 mass %.

It is possible to suppress propagation of cracks as compared with a spherical recrystallized structure by suppressing recrystallization and forming a recrystallized structure that extends in the extrusion axis direction, and ductility and formability are improved.

If the Fe content exceeds 0.40 mass %, a large amount of intermetallic compounds may be crystallized during casting, whereby a decrease in formability may occur.

Mn and Zr

The Mn content and the Zr content are preferably set to 0.3 mass % or less and 0.1 mass % or less, respectively.

Mn and Zr have an effect of producing an extruded metal structure in which the crystal grains are refined to improve formability and ductility. In the invention, Mn and Zr are not essential components.

It is possible to achieve sufficient quenching through fan air-cooling immediately after extrusion without increasing quench sensitivity as compared with Cr (transition element).

In the invention, Cr is considered to be unavoidable impurities.

If the Mn content and the Zr content are too high, quench sensitivity may increase, and a decrease in strength and formability may occur.

Therefore, the Mn content is preferably set to 0.01 to 0.3 mass %, and the Zr content is preferably set to 0.01 to 0.10 mass %.

Zn

Zn increases strength and ductility without impairing formability and extrudability. If the Zn content is high, however, a decrease in corrosion resistance may occur.

In the invention, Zn is not an essential component. When Zn is added to the aluminum alloy, the Zn content is set to 0.01 to 2.0 mass %, and preferably 0.02 to 1.50 mass %.

Ti

The Ti content is preferably set to 0.005 to 0.1 mass %.

Ti has an effect of refining the crystal grains during casting. If the Ti content is high, however, a number of coarse intermetallic compounds may be produced, and a decrease in strength may occur.

In the invention, the quality of the material is not significantly affected when the content of each component that is considered to be unavoidable impurities is 0.05 mass % or less, and the total content of components that are considered to be unavoidable impurities is 0.15 mass % or less.

Note that the content of each component that is considered to be unavoidable impurities is preferably 0.01 mass % or less, and the total content of components that are considered to be unavoidable impurities is preferably 0.10 mass % or less.

The aluminum alloy extruded material according to the invention exhibits excellent formability even during cold working through an air-cooling press quenching treatment performed immediately after extrusion, and exhibits high strength with a tensile strength of 245 MPa or more through subsequent artificial aging.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates the composition of each aluminum alloy used for evaluation.

FIG. 2 illustrates the billet casting conditions and the subsequent production conditions.

FIG. 3 illustrates the evaluation results for each extruded material.

DESCRIPTION OF EMBODIMENTS

Molten metal of each aluminum alloy having the composition listed in FIG. 1 (table) was prepared, and cast to obtain a columnar billet.

The casting speed is listed in FIG. 2 (table).

The billet was homogenized under the HOMO conditions listed in FIG. 2 (table).

Note that the material of Comparative Example 15 is a rolled material.

The billet was preheated to the BLT temperature listed in FIG. 2 (table), extruded at the extrusion speed listed in FIG. 2 (table), and air-cooled (subjected to press quenching) at the cooling rate listed in FIG. 2 (table) immediately after extrusion.

The air-cooled product was subjected to artificial aging under the heat treatment conditions listed in FIG. 2 (table).

The evaluation results for the extruded materials thus obtained are listed in FIG. 3 (table).

The extruded materials subjected to the evaluation were sheet materials having a thickness of 2.0 mm.

Each item was evaluated as described below.

Tensile Properties

A JIS No. 4 tensile specimen was prepared from the extruded material in accordance with JIS Z 2241. The specimen was subjected to a tensile test using a tensile tester conforming to the JIS standard.

Impact Resistance

A JIS V-notch No. 4 specimen was prepared from the extruded material in accordance with JIS Z 2242. The specimen was subjected to a Charpy impact test using a Charpy impact tester conforming to the JIS standard.

An extruded material having a thickness sufficient to prepare JIS V-notch No. 4 specimen was used.

Crystal Grain Size

A test material was mirror-polished and etched (3% NaOH, 40° C.×3 min). The metal structure of the test material was then observed using an optical microscope at a magnification of 100.

The aspect ratio (L1/L2) of the test material was measured. Note that the test material has the recrystallized structure that extends in the extrusion axis direction. L1 is the length of the crystal grain of the recrystallized structure in the extrusion direction while L2 is the length of the crystal grain in the thickness direction. Erichsen value

A sheet material (90 mm×90 mm×2 (thickness) mm) was prepared from the extruded material in accordance with JIS Z 2247. The specimen was subjected to an Erichsen test prior to artificial aging.

Specifically, a steel ball having a diameter of 20 mm was pressed into the surface of the sheet material, and the punch stroke was taken as the Erichsen value when a crack reached to the back side of the sheet material.

The larger the Erichsen value, the better the formability.

n-Value (Work Hardening Index)

The term “n-value” used herein refers to an exponent n when a true stress-true strain curve determined by a load-elongation curve is approximately represented by σ=Fε^(n).

The n-value corresponds to the slope of the line in which the true stress-true strain value is plotted into a double logarithmic graph.

Note that a JIS No. 4 tensile specimen was prepared from the extruded material in accordance with JIS Z 2241, and subjected to a tensile test using a tensile tester conforming to the JIS standard when measuring the n-value.

r-Value (Lankford Value)

The r-value (Lankford value) is the ratio of the true strain in the widthwise direction to the true strain in the thickness direction of the specimen during the tensile test (see below).

r=(ln w0/w1)/(ln t0/t1)

Note that w0 and w1 are the width of the specimen before and after the test, and t0 and t1 are the thickness of the specimen before and after the test.

Note that a JIS No. 4 tensile specimen was prepared from the extruded material in accordance with JIS Z 2241, and subjected to a tensile test using a tensile tester conforming to the JIS standard when measuring the r-value.

In Examples 1 to 5, the T1 tensile strength was 200 MPa or more, the T1 yield strength was 80 MPa or more, the T5 tensile strength was 245 MPa or more, the T5 yield strength was 205 MPa or more, and the Erichsen value was 11.0 or more (i.e., high strength and excellent formability were obtained) (see FIG. 3 (table)).

The n-value was 0.30 or more, and the r-value was 0.40 or more.

The elongation of the T1 material was 24% or more, and the elongation of the T5 material was 8% or more.

A Charpy impact value of 20 J/cm² or more was obtained while achieving high strength.

In Examples 1 to 5, the microstructure was a flat recrystallized structure, the aspect ratio was 4.0 or more, and the average crystal grain size was 100 μm or less.

In Comparative Example 11, since the Si content was 0.57 mass %, and the excess Si content (exSi) was 0.29 mass % (i.e., ≤0.30 mass %), the Erichsen value before the T5 treatment was small, and the T5 tensile strength was 200 MPa (i.e., ≤245 MPa).

In Comparative Examples 12 to 14, the excess Si content was lower than that of Comparative Example 1, and the Erichsen value was small although the strength was equal to or higher than the target value. Comparative Example 15 is a rolled material that differs from extruded material of Examples 1 to 5. In Comparative Example 15, the T5 yield strength was 122 MPa that was extremely lower than the target value of 205 MPa.

INDUSTRIAL APPLICABILITY

Since the extruded material produced using the aluminum alloy according to the invention exhibits excellent formability and the like, the extruded material may be used to produce various pressed products, bent products, and the like. 

What is claimed is:
 1. An aluminum alloy that is used to produce a high-strength aluminum alloy extruded material that exhibits excellent formability, the aluminum alloy consisting of 0.30 to 1.00 mass % of Mg, 0.6 to 1.40 mass % of Si, 0.10 to 0.40 mass % of Fe, 0.10 to 0.40 mass % of Cu, 0.005 to 0.1 mass % of Ti, 0.3 mass % or less of Mn, 0.01 to 2.0 mass % of Zn, and 0.10 mass % or less of Zr, with the balance being aluminum and unavoidable impurities, the aluminum alloy having a stoichiometric Mg₂Si content of 0.60 to 1.30 mass % and an excess Si content of 0.30 to 1.00 mass %.
 2. A high-strength aluminum alloy extruded material that exhibits excellent formability, the extruded material being produced using the aluminum alloy as defined in claim 1, the extruded material having a structure in which crystal grains having an aspect ratio of 4.0 or more have an average grain size of 100 μm or less.
 3. A method for producing a high-strength aluminum alloy extruded material that exhibits excellent formability, the method comprising extruding a billet of the aluminum alloy as defined in claim 1 to obtain an extruded product, air-cooling the extruded product immediately after the extrusion at an average cooling rate of 50 to 150° C./min, and subjecting the cooled extruded product to artificial aging. 